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Felix Flicker Email address for Felix Flicker

Research

Orcid ID: 0000-0002-8362-1384

Scopus Author ID: 56102472300

Please see the Publications tab for an up-to-date list of my papers, theses, etc.; this page has some popular write-ups of work I've been involved with.

Magnetic Monopoles in Spin Ice

Published in Nature.

My experimental collaborators detected signatures of magnetic monopoles in the spin ice material dysprosium titanate, based on our earlier theoretical predictions. You can read a popular summary of the work here:

a number of press releases here:

and an article about the original proposal here:

Further information from my talk “How to Pull The North Pole Off a Magnet”

I have given a few talks on this work to general audiences. Here are some references to material I mentioned in those talks.

For Paul Dirac's original paper on the quantum theory of magnetic monopoles see here. Both String Theories and Grand Unified Theories necessarily contain magnetic monopoles.

The original paper proposing magnetic monopoles in spin ices can be found here. The term 'magnetricity' was coined here.

I stated that, when asked what use electromagnetism has, Faraday said “What use is a newborn baby?”. I also mentioned the story is slightly apocryphal as he actually made the statement about a different subject. To see a full discussion see here.

The picture of artificial spin ice I presented was taken from this paper. I claimed that artificial spin ices are able to carry out classical computation close to the Landauer limit. To read more about this see the work of Prof. Laura Heyderman here.

To see how high-sensitivity measurements of magnetic fields can lead to efficient and less-intrusive MRI scanning see this paper.

The Colours of Noise

The measurement hinged on detecting the 'colour' of the noise present in the samples' fluctuating magnetic fields.

My former student Leon Zaporski has created an interactive program to demonstrate the colours of noise.

To use the interactive noise program, click here.

White noise contains an equal amount of power over any equal frequency interval. It is named in analogy to white light, made from an equal mix of all colours. The noise spectral density as a function of frequency, S(f), is a constant.

Pink noise, on the other hand, has a noise spectral density inversely proportional to the frequency. There is equal energy contained in each octave. At higher frequencies (blue, by analogy to light) there is less intensity, so the result is 'pink' overall (more weight at low frequencies). While white noise is generally considered to be quite unpleasant to hear, pink noise is used to test speaker cabinets during their manufacture, as it gives a good approximation to the distribution of sounds appearing in music.

Red noise, also called Brownian noise, has a noise spectral density inversely proportional to the frequency squared. The drop-off in noise spectral density occurs more rapidly than in pink noise, and there is even more weight at the red (low) frequencies. This is the noise spectrum you would detect from a particle undergoing Brownian motion or a random walk; it is also the noise spectrum the monopole experiment would have detected if spin ices were standard paramagnets, as the same crystals are known to be at higher temperatures.

There are forms of noise with more weight at high frequencies. Blue noise obeys has a noise spectral density proportional to the frequency: the blue light characteristic of Cherenkov radiation is a good example. Violet noise has a noise spectral density proportional to the frequency squared.

In the recent experiment, the noise detected by the SQUID varied between pink and red, becoming pinker with increasing temperature. In fact, since the sensitivity of the SQUID overlaps with that of human ear, by creating a sound with the same mix of frequencies as the flux measurement, you can hear the noise of the monopoles directly.

The following audio file, recorded during the experiment, features three seconds of the noise recorded before the spin ice sample is inserted into the coil (approximately white), followed by three seconds of the noise after the sample is inserted (pink-to-red). The intensity of the noise also jumps significantly (another key prediction verified in the experiment), although the signals here have been normalised for listenability. You are listening to the first recording of magnetic monopoles in spin ice!

To listen to the audio file, click here.

The Excitonic Insulator State in Titanium Diselenide

Published in Science

Charge Density Waves in Niobium Diselenide

Some press releases by my collaborators: